The present invention relates generally to apparatus and methods for precision analysis of gaseous systems, and more particularly to a triple-pump coherent anti-Stokes Raman scattering (CARS) system for simultaneous measurements of temperature and species concentrations with high spatial and temporal resolution.
Complete characterization of reacting gaseous flow systems requires a determination of performance and chemical efficiency by measuring temperature and species concentrations either in the reaction zone or in the exhaust stream. Typically, these data are acquired utilizing several independent techniques executed serially, but the measurements would ideally be made simultaneously with a minimum number of diagnostic measurements.
Previously known CARS techniques have been used for temperature and multiple-species-concentration measurements, such as dual broadband rotational CARS (Martinsson et al, “Oxygen Concentration and Temperature Measurements in N2—O2 Mixtures using Rotational Coherent Anti-Stokes Raman Spectroscopy,” Applied Physics B 62, 29–37 (1996); Thumann et al, “Simultaneous Temperature and Relative Nitrogen-Oxygen Concentration Measurements in Air with Pure Rotational Coherent Anti-Stokes Raman Scattering for Temperatures to as High as 2050 K,” Applied Optics 36, 3500–3505 (1997); and Bood et al, “Temperature and Concentration Measurements in Acetylene-Nitrogen Mixtures in the Range 300–600 K using Dual-Broadband Rotational CARS,” Applied Physics B 70, 607–620 (2000)) or simultaneous vibrational and rotational CARS (Yueh et al, “Simultaneous N2, CO, and H2 Multiplex CARS Measurements in Combustion Environments Using a Single Dye Laser,” Applied Optics 27, 3233–3243 (1988); Bengtsson et al, “Combined Vibrational and Rotational CARS for Simultaneous Measurements of Temperature and Concentration of Fuel, Oxygen, and Nitrogen,” Applied Spectroscopy 49, 188–192 (1995); Seeger et al, “Experimental Comparison of Single-Shot Vibrational and Dual-Broadband Rotational Pure Rotational Coherent Anti-Stokes Raman Scattering in Hot Air,” Applied Optics 35, 2665–2671 (1996); and Brackmann et al, “Simultaneous Vibrational and Pure Rotational Coherent Anti-Stokes Raman Spectroscopy for Temperature and Multispecies Concentration Measurements Demonstrated in Sooting Flames,” Applied Optics 41, 564–572 (1996)). However, the accuracy of the temperatures derived from rotational CARS measurements decreases with increasing temperature (Aldén et al, “Rotational CARS: a comparison of different techniques with emphasis on accuracy in temperature determination,” Applied Optics 28, 3206–3219 (1989)).
The dual-pump CARS technique, first demonstrated by Lucht et al (“Measurements of Temperature and CO2 Concentrations by Dual-Pump Coherent Anti-Stokes Raman Scattering,” AIAA Journal 41, 679–686 (2003)) has been used for the simultaneous measurement of N2/O2 (see Lucht, “Three-Laser Coherent Anti-Stokes Raman Scattering Measurements of Two Species,” Optics Letters 12, 78–80 (1987) and Hancock et al, “Dual-Pump Coherent Anti-Stokes Raman Scattering Measurements of Hydrogen and Oxygen in a Laminar Jet Diffusion Flame,” Applied Optics 36, 3217–3226 (1997)), N2/H2 (Schauer, “Investigation of Flame Structure and Thermal Diffusion Effects in Hydrogen Jet Diffusion Flames,” Ph.D. Thesis, Univ. of Illinois, Urbana Ill. (1998)), N2/CH4 (Green et al, “An Annular Phase-Matched Dual-Pump CARS System for the Simultaneous Detection of Nitrogen and Methane,” Applied Optics 37, 1690–1701 (1998)), and N2/CO2 (Brüggemann et al, “CARS Spectroscopy for Temperature and Concentration Measurements in a Spark Ignition Engine,” in Combustion Flow Diagnostics, by Duräom et al, eds., Kluwer Academic, Dordrecht, The Netherlands, 495–511 (1992)).
In a dual-pump CARS system, the wavelengths of the input beams are adjusted such that the CARS spectra for the two species under study are observed at nearly the same frequency. This arrangement largely eliminates the potential errors arising from wavelength-dependent variations in signal transmission or detector efficiency that can complicate other multiple-species CARS techniques such as dual-Stokes and dual-broadband CARS (Eckbreth, Laser Diagnostics for Combustion Temperature and Species, Gordon and Breach Publishers, Amsterdam, The Netherlands, (1988)).
The invention described and claimed herein solves or substantially reduces in critical importance problems with prior art techniques by providing a triple-pump CARS system for accurate measurement of temperature along with concentration of three species in high-temperature reacting flows. The invention permits monitoring of the local temperature and concentrations of two target species with respect to a reference species using a single hardware platform with high spatial and temporal resolution.
A triple-pump CARS system representative of the invention uses four laser beams to generate CARS signals near two distinct wavelengths. Both wavelength regions exhibit an N2 CARS signal along with the CARS signal from another target molecule. Each pair of CARS signals is generated over a relatively narrow wavelength region and can be captured with fixed-wavelength detection. Temperature and relative concentrations of the target species (with respect to N2) are extracted by fitting the measured CARS spectrum in each wavelength region. Because the N2 concentration is typically initially known, the amplitude of the target-species signal relative to that of the paired N2 signal provides an absolute measure of the target-species concentration. In demonstration of the invention, tests were performed in an atmospheric-pressure hydrogen-air diffusion flame, and the CARS signals from N2/O2 and N2/H2 pairings were acquired using two spectrometers each equipped with a charged coupled device (CCD) camera. For measurements made in reacting flows using air as the oxidizer, nitrogen is the most convenient reference molecule. Nitrogen CARS spectra are known over a very wide range of temperature and pressure.
It is a principal object of the invention to provide a system for analysis of gaseous systems.
It is a further object of the invention to provide an improved system and method for precision analysis of flowing gaseous systems.
It is another principal object of the invention to provide a triple-pump CARS system for simultaneous measurements of temperature and species concentrations with high spatial and temporal resolution.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.